Collision avoidance for ASVs through trajectory planning: MPC with COLREGs-compliant nonlinear constraints

Abstract

This article presents a trajectory planning method for autonomous surface vessels that is compliant with Rule 8 and rules 13-17 from the Convention on the International Regulations for Preventing Collisions at Sea (COLREGs). The method is suitable for operation in restricted waters, where it both handles collision avoidance with static obstacles, and also considers the available room to maneuver when determining the appropriate safe distance to other vessels. The trajectory planner is formulated as a finite-horizon nonlinear model predictive controller, minimizing the deviation from a reference trajectory and the acceleration. Collision avoidance with static obstacles is included through the use of convex free sets. Collision avoidance with other traffic is done by assigning so-called target ship domains to each vessel, and formulating constraints for that domain. COLREGs rules 13-15 and 17 are included by first classifying each vessel-to-vessel encounter to find which rule applies, and subsequently assigning an encounter-specific domain to the opposing vessel. The domain is designed so that if the trajectory does not violate the domain, compliance with COLREGs rules 13-15 and partial compliance with Rule 17 is ensured. Furthermore, compliance with COLREGs Rule 8 and Rule 16 is included through a novel method for calculating the objective function cost-gains. By constructing windows of reduced tracking error and acceleration cost, the start time, duration and magnitude of a maneuver can be controlled, and hence readily apparent maneuvers made in ample time can be facilitated. The method's effectiveness and its completeness in terms of COLREGs compliance is demonstrated through an extensive set of simulations of vessel-to-vessel encounters in open waters. Furthermore, the robustness of the method is demonstrated through a set of complex simulations in confined areas with several maneuvering vessels. In all simulations, the method demonstrates compliance with COLREGs Rule 8 and rules 13-17.